The power split hybrid electric vehicle (HEV) is a parallel hybrid electric vehicle. FIG. 1 shows the power split HEV powertrain configuration and control system. In this powertrain configuration, there are two power sources that are connected to the driveline: 1) a combination of engine and generator subsystems using a planetary gear set to connect to each other, and 2) the electric drive system (motor, generator, and battery subsystems). The battery subsystem is an energy storage system for the generator and the motor. FIG. 2 shows the possible power flows in this powertrain configuration.
In the first power source, as shown in FIG. 2, the engine output power can be split into two paths by controlling the generator-mechanical path trωr (from the engine to the carrier to the ring gear to counter shaft), and electrical path τgωg to τmωm (from the engine to the generator to the motor to the counter shaft). The way to split the engine power is to control the engine speed to a desired value, which results in a definite generator speed for a given ring gear speed, (or vehicle speed), because of the kinematics property of a planetary gear set.
The generator speed will change according to the vehicle speed for a definite engine speed, and the engine speed can be decoupled from the vehicle speed. The changing generator speed will vary the engine output power split between its electrical path and mechanical path. In addition, the control of engine speed results in a generator torque to react against the engine output torque. It is this generator reaction torque that conveys the engine output torque to the ring gear of the planetary gear set, and eventually to the wheels. This mode of operation is called “positive split”. It is noted that because of the mentioned kinematics property of the planetary gear set, the generator can possibly rotate in the same direction of its torque that reacts against the engine output torque. In this operation, the generator inputs power (like the engine) to the planetary gear set to drive the vehicle. This operation mode is called “negative split”. As in the case of the positive split mode, the generator torque resulting from the generator speed control reacts to the engine output torque and conveys the engine output torque to the wheels. Clearly, this combination of the generator, motor and planetary gear set is analogous to an electro-mechanical CVT. When the generator brake (shown in FIG. 1) is actuated (parallel mode operation), the sun gear is locked from rotating and the generator braking torque provides the reaction torque to the engine output torque. In this mode of operation, all the engine output power is transmitted, with a fix gear ratio, to the drivetrain through the mechanical path only.
In the second power source, the electric motor draws power from the battery and provides propulsion independently from the engine to the vehicle for forward and reverse motions. This operating mode is called “electric drive.” In addition, the generator can draw power from the battery and drive against a one-way clutch coupling on the engine output shaft to propel the vehicle forward. The generator can propel the vehicle forward alone when necessary, and this mode of operation is called generator drive mode.
Operation of the power split powertrain system, unlike conventional powertrain systems, attempts to integrate the two power sources to work together seamlessly to meet driver demand without exceeding system limits (such as battery limits) while optimizing the total powertrain system efficiency and performance. Control between the two power sources is coordinated. As shown in FIG. 1, there is a hierarchical vehicle system controller (VSC) that performs the coordination control in this power split powertrain system. Under normal powertrain conditions (no subsystems/components faulted), the VSC interprets the driver's demands (e.g. PRND and acceleration or deceleration demand), and then determines the wheel torque command based on the driver demand and powertrain limits. In addition, the VSC determines when and how much torque each power source provides to meet the driver's torque demand and achieve the operating point (torque and speed) of the engine.